In a typical "valveless" DTH drill, the passage of compressed air through the drill assembly reciprocates a piston which repeatedly strikes a bit, wherein pressure air is alternately supplied to either side of the piston by a porting arrangement defined by air passageways in an internal sleeve in the cylinder, without the use of a moving valve such as a flapper valve. Pressure air is exhausted through a central bore in the piston and bit to the bore hole being drilled, which carries debris to the surface in an annular cavity surrounding the drill string. The central bore must be closed to allow the return stroke of the piston to commence, for example by means of a short tube or "footvalve" secured to the innermost end of the bit extending from the bit into the central bore of the piston, so that when the piston approaches the striking face of the bit on the downstroke the tube penetrates the central bore of the piston to seal the central bore.
U.S. Pat. No. 4,084,646 describes a high output valveless DTH drill, in which the high output performance is achieved by maintaining a continuous supply of live pressure fluid on rearwardly-facing surfaces defined between the ends of the piston by shoulders joining a smaller diameter portion of the piston with a larger diameter portion, thereby providing a constant driving force on the piston in the direction of the bit throughout the operating cycle. This constant force supplements the cyclic force applied to the rear end pressure surface, or crown, of the piston when the rear chamber is pressurised, so that the average force against the piston on its drive stroke is higher than previously known drills which only exhaust pressure fluid during the latter phase of the drive stroke. The piston is designed to have a circumferential "sealing" surface with the cylinder sleeve at its rear end adjacent to the crown, and with the cylinder casing or wear sleeve at its front end as defined by the circumference of a "clubhead" portion, so that a single chamber is defined between the cylinder sleeve, the cylinder casing and the reduced diameter portions of the piston between the two circumferential "sealing" surfaces. During operation, this chamber is constantly in fluid communication with the pressure fluid source, that is, it forms a reservoir of live pressure fluid. This can either enter a chamber rearwardly of the crown of the piston or a chamber forwardly of the striking or hammer surface of the piston, so as to reciprocate the piston within the cylinder, but will always exert a force upon the rearwardly facing shoulders o the piston.
In the efforts to attain better output performance, higher penetration rates and greater depth of penetration in difficult drilling conditions, there has been a tendency towards higher applied pressure, but workers skilled in the art have not recognised the full significance of the loss of effective working pressure through leakage past the very small gaps necessary for a sliding fit clearance between parts of the drill.
The valveless DTH drill cycle described in U.S. Pat. No. 4,084,646 will be generally effective at relatively low pressure (100 psi to 250 psi) operation and in the absence of any significant leakage of pressure fluid which can occur (a) between the head of the piston and the cylinder casing or wear sleeve, and (b) between the shank of the bit and the bit-retaining bearing. However, in DTH drills constructed with normal running clearances of typically 0.003-0.006 inches (0.07-0.15 mm) between the piston and cylinder and where the applied pressure is relatively high (above 250 psi) tests have shown that leakage past the head of the piston is of such a magnitude as to cause a substantial drain on the pressure fluid supply and so reduce the effective working pressure of the piston. The pressure fluid which may leak past the piston also has the effect of charging the exhaust passages with an excess of pressure fluid, which further reduces the performance of the hammer drill. The clearance between the bit-retaining bearing and the shank of the bit is typically 0.014-0.019 inches (0.25-0.45 mm) which appears to be such as to cause a very substantial amount of leakage of pressure fluid from the chamber forward of the striking surface of the piston. This reduces the lifting force on the piston at the beginning of the return stroke so as to shorten the stroke and thereby reduce the blow energy of the piston on its drive stroke.
It is an object of the present invention to provide a high performance valveless DTH hammer drill operating with a cycle which provides additional impetus to the piston on its drive stroke without continuously supplying live pressure fluid to the piston, such that loss of working pressure by leakage through clearance gaps is kept to a minimum. It is also an object of the invention to provide better sealing arrangements in a valveless DTH hammer drill adapted to operate at high pressure, i.e above 250 psi.